While weather is complex and unpredictable, economic systems are even more so. Does that mean there are no principles of economics? Of course not. In fact, it’s because of such complexity that we need whatever help economic theory can offer to organize our thinking. And it doesn’t get any more basic than this: The demand curve for goods slopes downward.

That is, other things equal, the costlier something is, the less of it you’ll want to buy.

Note that the caveat—other things equal—is as important as the inverse relation between price and quantity demanded. That’s why my earlier back-of-the-envelope analysis had to be conditional on more data. Unfortunately, those data are often very hard to get. Does that mean we abandon the theory? Well, that would be like letting go of the rope you’re hanging on to for dear life because you’re afraid it might break.